import java.util.Scanner;

/**
 * Created with IntelliJ IDEA.
 * Description:
 * User: 洪汤泽大帅哥
 * Date: 2024-11-18
 * Time: 22:29
 */
/* 请在这里填写答案 */

class NotSanjiaoException extends Exception{
    public NotSanjiaoException(String string){
        super(string);
    }
}
class Sanj {
    private double a;
    private double b;
    private double c;

    Sanj(double a, double b, double c) throws Exception{
        if (a+b>c && a+c>b && b+c>a ){
            this.a = a;
            this.b = b;
            this.c = c;
        }
        else{
            throw new NotSanjiaoException("三边长不能构成三角形");
        }
    }

    public double getArea() {
        return a*b/2.0;
    }
}


public class S1 {
    public static void main(String args[]) {
        try {
            Scanner scan = new Scanner(System.in);
            double a, b, c;
            a = scan.nextDouble();
            b = scan.nextDouble();
            c = scan.nextDouble();
            Sanj sj = new Sanj(a, b, c);
            System.out.println("三角形面积为：" + sj.getArea());
        } catch (Exception e) {
            System.out.println(e);
        }
    }
}